The Guaiac Fecal Occult Blood Test (gFOBT) is a non-invasive method used to detect hidden blood in the stool, known as “occult” blood. This at-home test is one of the earliest forms of stool-based screening for gastrointestinal issues. It involves a chemical reaction on a small card to indicate the presence of blood that is not visible to the naked eye. The test provides a straightforward tool for initial screening, which can then guide the need for more intensive diagnostic procedures.
Primary Medical Purpose
The main reason a doctor orders a gFOBT is for screening average-risk individuals for colorectal cancer. This type of cancer often begins as small growths called polyps, which can bleed intermittently as stool passes over them. Detecting this hidden blood is a way to find the cancer or precancerous polyps early, often before any noticeable symptoms appear.
Finding blood in the stool does not mean a person has cancer, as the bleeding can come from many other sources within the digestive tract. Other common causes of gastrointestinal bleeding detectable by this test include ulcers, hemorrhoids, diverticulosis, or inflammatory bowel disease. The gFOBT acts as a filter, indicating a need for further investigation to pinpoint the exact source of the bleeding.
The Chemical Reaction Behind the Test
The gFOBT relies on a chemical principle called the pseudo-peroxidase activity of hemoglobin, the protein in red blood cells. The test kit uses filter paper impregnated with a substance called guaiac, which is derived from the resin of the guaiacum tree. When a stool sample is applied to the paper, the test is developed by adding a solution containing hydrogen peroxide.
If blood is present in the stool, the iron-containing heme component of the hemoglobin acts as a catalyst, accelerating the reaction between the guaiac and the hydrogen peroxide. This rapid oxidation converts the colorless guaiac into a blue-colored quinone compound. A positive result is indicated by the appearance of this distinct blue color on the test card within a few seconds.
Patient Preparation and Sample Collection
Following specific preparation instructions is important for the gFOBT to ensure accurate results. Certain foods and medications can cause false results by interfering with the chemical reaction. For three days before and during the testing period, patients must avoid red meat, as the animal’s blood can cause a false positive result due to the presence of heme.
Patients also need to restrict their intake of certain raw vegetables and fruits, such as turnips, radishes, horseradish, and cantaloupe, which contain plant peroxidases that can similarly trigger a false positive. Conversely, consuming high doses of Vitamin C (typically more than 250 mg daily) can prevent the chemical reaction from occurring, leading to a false negative result even if blood is present. Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin or ibuprofen should also be avoided for about seven days before the test, as they can cause minor gastrointestinal bleeding and lead to a false positive.
The home collection process typically involves gathering samples from three separate bowel movements over three consecutive days. Using a small applicator, a thin smear of stool is applied to two different areas on the test card for each of the three samples. This repeated collection process increases the chance of detecting blood, as gastrointestinal bleeding is often intermittent.
Understanding Test Results
The results of the gFOBT are reported as either positive or negative. A negative result means no occult blood was detected in the tested samples, suggesting no active bleeding in the digestive tract at the time of collection. A positive result indicates the presence of hidden blood, meaning a source of bleeding exists somewhere in the gastrointestinal system.
It is important to understand that a positive gFOBT result is not a diagnosis of cancer; it simply confirms the presence of blood. Other conditions, such as hemorrhoids or ulcers, are more common causes of a positive result than cancer. The test’s sensitivity for detecting colorectal cancer is moderate, ranging from 50% to 70%, but its specificity, or ability to correctly identify the absence of blood, is high.
If the test is positive, the next step is usually a follow-up diagnostic procedure, most commonly a colonoscopy. This visual examination allows a doctor to determine the precise location and cause of the bleeding. A negative test is considered normal, but screening should be repeated annually as recommended by a healthcare provider.